Asymmetric Segmented Channel Transistors

ABSTRACT

Structures, layouts and methods of forming integrated circuits are described. In various embodiments, the current invention includes an asymmetric segmented transistor. The asymmetric segmented transistor includes a source region and a drain region disposed within an active region, a floating source/drain region disposed within the active region, a first channel region disposed in the active region between the source region and the floating source/drain region, the first channel having a first length and a first width. A second channel region is disposed in the active region between the drain region and the floating source/drain region, the second channel having a second length and a second width. A first gate dielectric overlies the first channel region and a second gate dielectric overlies the second channel region. A gate line overlies the first gate dielectric and the second gate dielectric.

This application is a divisional of U.S. patent application Ser. No.12/036,911 filed on Feb. 25, 2008, which application is incorporatedherein in entirety by reference.

TECHNICAL FIELD

This invention relates generally to electronic devices, and inparticular embodiments to asymmetric segmented channel transistors.

BACKGROUND

Semiconductor devices are used in a large number of electronic devices,such as computers, cell phones and others. One of the goals of thesemiconductor industry is to continue shrinking the size and increasingthe speed of individual devices. Every generation of semiconductordevices is smaller than the previous generation by about 50%, thusreducing the cost per device significantly.

A key challenge in scaling semiconductor devices arises due to theincrease in leakage currents at short channel lengths. Halo implants areused to mitigate this increase leakage arising from short channeleffects. However, use of halo implants increases the channel doping aswell counter-dopes the source and drain junctions. Both these effectsincrease the effective resistance of the transistor. However, shortchannel devices compensate some of this loss by gate length scalingaccompanying the technology development. However, longer gate lengthtransistors built on such short channel device technologies do not enjoythis advantage and are hence degraded. This is often not a problem indigital technologies that use only the short gate length devices.

Analog circuits typically employ longer gate length devices and cannottake advantage of these aggressively scaled transistors. For example,these short channel digital devices have lower analog intrinsic gain aswell as increased gate leakages. Due to cost constraints, analogtechnologies utilize the CMOS technologies developed for digitalapplications. As the long gate lengths devices are also degraded, manyanalog applications require individually tailored technology flows,considerably increasing production costs.

Hence, improved methods, structures of forming and designing analogdevices are needed that are compatible with existing CMOS technologies.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by embodiments of thepresent invention which provide structure, methods and circuitscomprising asymmetric devices.

Embodiments of the invention include methods, structures and layouts forasymmetric segmented transistors in integrated circuits. In accordancewith an embodiment of the present invention, a mixed signal integratedcircuit includes an asymmetric segmented transistor comprising a sourceregion and a drain region disposed within an active region, a floatingsource/drain region disposed within the active region, a first channelregion disposed in the active region between the source region and thefloating source/drain region, the first channel having a first lengthand a first width. A second channel region is disposed in the activeregion between the drain region and the floating source/drain region,the second channel having a second length and a second width. A firstgate dielectric overlies the first channel region and a second gatedielectric overlies the second channel region. A gate line overlies thefirst gate dielectric and the second gate dielectric.

The foregoing has outlined rather broadly the features of embodiments ofthe present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates the key analog device parameters for different gatelength transistors;

FIG. 2 illustrates the key analog device parameters for a given deviceor gate length;

FIG. 3, which includes FIGS. 3 a-3 c, illustrates a structuralembodiment of the invention, wherein FIGS. 3 a and 3 b illustrate topcross sections and FIG. 3 c illustrates a cross sectional view;

FIG. 4, which includes FIG. 4 a-4 c, compares the key analog deviceparameters from the structural embodiment described in FIG. 3 for use inhigh precision analog circuits;

FIG. 5 compares the key analog device parameters from the structuralembodiment described in FIG. 3 for use in high performance analogcircuits;

FIG. 6, which includes FIGS. 6 a-6 f, illustrates a top view ofstructural embodiments of the invention;

FIG. 7 is a flow chart of a method of forming the analog devicesaccording to an embodiment of the invention;

FIG. 8, which includes FIGS. 8 a-8 e, illustrates cross sections of thedevice during various stages of the manufacturing illustrated in flowchart 7;

FIG. 9, which includes FIGS. 9 a and 9 b, wherein FIG. 9 a illustratesan embodiment of a mixed signal chip comprising analog devices and logicdevices and FIG. 9 b illustrates the threshold voltages used in theanalog and logic devices in accordance with embodiments of theinvention;

FIG. 10, which includes FIGS. 10 a and 10 b, wherein FIG. 10 aillustrates an embodiment of a mixed signal chip comprising analogdevices and logic devices, the analog devices comprising at least onedevice with multiple segments, and FIG. 10 b illustrates the thresholdvoltages used in the analog and logic devices in accordance withembodiments of the invention;

FIG. 11, which includes Figures 11 a-11 c, illustrates two-stage CMOSamplifier circuits incorporating embodiments of the invention; and

FIG. 12, which includes FIG. 12 a and FIG. 12 b, illustrates currentmirrors, in accordance with embodiments of the invention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely as an asymmetric segmentedchannel transistor. Embodiments of the present invention includemethods, structure, layouts, and circuits comprising asymmetricsegmented channel transistor. The invention may also be applied,however, to other semiconductor components comprising, for example,other non CMOS devices. One of ordinary skill in the art will be able torecognize further examples as well.

Digital applications primarily drive CMOS scaling, in which theobjectives are different—smaller, faster, and lower-power devices.Extremely scaled CMOS devices of about 30 nm are already in production,and further smaller devices are being explored. These small devicesexhibit excellent speed (drive currents) but some of this performance isat the expense of much higher device off-current, about 100 nA/μm at 25°C., significantly compromising the low-standby-power attribute of CMOStechnologies. To maintain enough gate current control, these extremelyscaled devices typically use ultra-thin gate oxides (1.5 nm or thinner),resulting in gate tunneling current density as large as 10−100 A/cm².However, devices used in analog circuits cannot handle such large gatecurrents. Typically, this is not a problem as CMOS technologies offermultiple gate oxide technologies.

However, the speed (drive current) of the device is not the only Figureof Merit (FOM) the analog designer has to deal with. Otherspecifications that are as important if not more are devicetransconductance (g_(m)), output conductance (g_(ds)), cutoff frequency(f_(T)), maximum frequency of oscillation (f_(max)), Vt matching, 1/fnoise and NF characteristics. Dynamic range is also important, alongwith other economic issues such as time to market, system specs andcost.

The small signal transconductance g_(m) is defined as change in draincurrent I_(ds) of a CMOS transistor for a given change in gate voltageV_(g) and hence g_(m)=dI_(ds)/dV_(g). The transconductance degrades withincrease in gate voltage V_(g) due to degradation in mobility arisingfrom increased surface roughness scattering. Again at high channeldopings, the transconductance degrades due to impurity induced coulomb(or impurity) scattering. In scaled CMOS devices, the channel doping ishigh due to the need for shielding source of the device from the drainof the device, or rather reduce the off current. Typically, haloimplants are used to control short channel effects. However, this halodoping also counter-dopes the source and drain regions. Consequently thesource and drain extension regions of the transistor are alsosignificantly degraded (reducing g_(m)) due to increased resistance fromthe counter doping of the halo implants. A long channel transistor builtwith such halo implants may not be as significantly impacted by thereduced g_(m) around the source/drain regions as the middle section ofthe long channel transistor has no halo doping.

The parameter output conductance g_(ds) relates the sensitivity of draincurrent to a change in drain voltage, and hence g_(ds)=dI_(ds)/dV_(ds).A small output conductance is desirable for many analog circuitapplications. However, high-speed CMOS devices usually have a largeoutput conductance. The large output conductance is a direct result ofthe fact that short-channel devices have a large drain-inducedbarrier-lowering effect which causes the drain current to increase withdrain voltage. Hence, as illustrated in FIG. 1, g_(ds) increases withgate length. When small output conductance is needed, designers oftenuse device channel lengths significantly larger than minimum for thetechnology. However, the additional halo dose added to control the draininduced barrier lowering also significantly degrades the outputconductance g_(ds). Hence, longer channel devices built on technologiesoptimized for short channel devices suffer from increased outputconductance g_(ds).

The primary analog device design strives to: (a) maximize speed of theamplifier (˜gm/C_(load)); (b) achieve g_(m) at lowest possible currentto minimize power consumption (hence maximize g_(m)/I_(ds)); (c)maintain accuracy (e.g. of the single stage amplifier) by maximizing thevoltage gain g_(m)/g_(ds;) (d) minimize device mismatch (e.g. V_(T))between identical devices; and (e) minimize noise (low frequency 1/f orflicker noise and high frequency noise such as NF noise).

A commonly used figure of merit for quantifying speed or g_(m)/C_(load)is the cutoff frequency (f_(T)), which is defined as the frequency atwhich the small signal current gain drops to unity. The cutoff frequencyis a measure of the maximum useful frequency of a transistor when it isused as an amplifier. Scaled CMOS devices can have very high cutofffrequency. However, the series resistances associated with thesource/drain and gate, together with the characteristic short-channeleffects associated with very-short-channel CMOS devices, limits theattainable maximum frequency of oscillation (f_(max)). The maximumfrequency of oscillation (f_(max)) is the frequency at which unilateralcurrent gain becomes unity, and is hence a more reliable metric. Thetypical scale of operation of an application is about 5-10 times lowerthan the f_(T).

Currently there is great interest in developing CMOS with reducedparasitic resistance, including using a metal as the gate electrode.With a metal gate and careful layout to minimize parasitic resistance,the attainable f_(max) could approach 300 GHz. However, another keymetric, intrinsic voltage gain g_(m)/g_(ds) is significantly degradedfor scaled CMOS devices (FIG. 1).

FIG. 2 illustrates the tradeoff in key parameters in designing analogdevices. For a given gate length device, a peak in f_(T) (curve 11) andf_(max) (curve 12). For example, f_(T) increases with source to draincurrent I_(ds) (or gate over drive). However, both g_(m)/g_(ds) (curve14) and g_(m)/I_(ds) (curve 13) degrade with increasing source to draincurrent I_(ds). At any given technology node, simultaneous maximizationof f_(T) and intrinsic voltage gain g_(m)/g_(ds) is not possible. Hence,analog devices are typically designed based on application i.e. highperformance analog technologies favoring higher speed, and precisionanalog technology favoring intrinsic voltage gain. Hence, independentlyoptimized devices are used to overcome these limitations. For example,devices with high intrinsic gain can be fabricated using long channellength with low channel doping. These devices may be suitably designedto maintain a high intrinsic voltage gain. However, as discussed thiscomes with a penalty in f_(T).

Hence, applications requiring high f_(T) may be custom designed on adifferent technology such as a BiCMOS process flow. However, rather thanimplement expensive solutions such as adopting an independent BiCMOSprocess flow, many applications require a low cost solution and insteadcompromise some of the device metrics. For example, a BiCMOS process maybe 30-40% more expensive than a corresponding CMOS process. Hence, formany applications devices are tailored for each application and do notutilize the product development and enhancements available in CMOSprocess flows. For example, such analog devices are designed withthicker oxides, longer channel lengths, and unique source/drainextensions. Consequentially, this process requires a custom CMOS flowand is still expensive for many applications.

In various embodiments, the present invention overcomes theselimitations by reducing the process complexity associated with formationof analog high performance and/or high precision CMOS devices. Invarious embodiments, the analog device comprises a combination of ashort gate length CMOS device and a long gate CMOS device into a singlestructure to tailor an analog device with suitable devicecharacteristics. In various embodiments, properties of a longer gatelength device are achieved by a suitable combination of smaller gatelength devices.

Structural embodiments of the invention will be first described usingFIGS. 3 and 6. Various embodiments of the method of fabrication willthen be described using the flow chart of FIGS. 7, and FIG. 8.Embodiments of manufacturing and chips manufactured with logic andanalog devices are illustrated in FIGS. 9 and 10. Finally, embodimentsof the invention applied in analog circuits will illustrated in FIGS. 11and 12.

An embodiment of the invention is illustrated in FIG. 3 which includesFIG. 3 a-3 c. FIGS. 3 a and 3 b illustrate the top cross sectional viewand FIG. 3 c illustrates a vertical cross sectional view of theasymmetric channel transistor.

FIG. 3 a illustrates a top cross section of the device illustrating alayout (e.g. regions in mask layers for forming regions) in anembodiment of the invention. The device is embedded in an isolationregion 40, which, for example, may be an STI region.

The silicon active region 20 is surrounded by the isolation region 40and contains a source 30 and a drain 60. The gate line 50 includes a “U”shaped region forming two distinct gate regions overlying the siliconactive region 20. The contacts 70, 75 and 80 are disposed on the drain60, gate line 50 and source 30 respectively. FIG. 3 b illustrates thetop cross section after formation of active regions. As illustrated inFIG. 3 b, the spacer 51 is disposed around the gate line 50. The gateline 50 and the spacer 51 overlie the isolation region 40 forming the“U” shaped region. FIG. 3 c illustrates the corresponding vertical crosssection of the device 10 disposed in a substrate 100. The isolationregion 40 of the device 10 comprises shallow trench isolation andtypically comprises a trench filled with an oxide material. The sourceis contacted via source contact 80 by connecting to a voltage node andapplies a first voltage. The second terminal or drain contact 70 isconnected to voltage node to apply a second voltage. The gate line 50 isconnected to a third voltage using gate contacts 75. The gate line 50includes a first gate 90 and a second gate 95. The first and second gate90 and 95 form corresponding first channel 91 and second channel 96underneath. The potential difference applied between the source contact80 and drain contact 70 determines the direction of the current flow(shown by the arrow). For the sake of illustration, a forward biaseddevice has a current flow as shown by the arrow in FIG. 3 c. A reversebiased device has the potential difference reversed and hence reversingthe direction of the corresponding flow of current. In all these casesthe gate contacts 75 (and substrate 100) are biased to form an inversionlayer in the first and second channels 91 and 96.

As illustrated in FIG. 3 a, the first gate 90 has a gate length L₁ andwidth W₁, and the second gate 95 has a gate length L₂ and width W₂. Invarious embodiments, the first gate 90 and second gate 95 comprisedifferent gate lengths. In preferred embodiments, the second gate lengthL₂ is about 1.5 to 2.5 times the length of the first gate length L₁, andthe first gate length L₁ is at the minimum gate length for the giventechnology. Typically but necessarily, the widths W₁ and W₂ are aboutequal.

In various embodiments, a method of optimization for determining thelengths and widths uses an optimization procedure to match the targetspecifications derived for an individual circuit. The device is tailoredfor each circuit by individually changing the lengths and widths of thedevices.

The performance of the device 10 (e.g. of FIG. 3 c) after theoptimization, in different embodiments is illustrated in FIGS. 4 and 5.FIGS. 4 a-4 c illustrate a precision analog device with optimizedintrinsic voltage gain. Curve 2 refers to the device in FIG. 1 biased inforward bias mode. Curve 3 refers to the device in FIG. 1 biased inreverse bias mode. Curve 1 refers to a standard symmetric device. Thestandard symmetric device is a longer channel device such that the gatelength of the standard device is longer than the sum of the first gatelength L₁ of the first gate and the second gate length L₂ of the secondgate. Using embodiments of the invention, curve 2 illustratesimprovements in g_(m) and g_(ds) (hence, the ratio g_(m)/g_(ds))obtainable over the standard device especially in bias conditionssuitable for analog operation (gate voltage is about 100 mV above thethreshold voltage). Even the reverse biased device shows respectabledevice characteristics.

FIG. 5 illustrates the embodiments of the invention applied forproducing a analog device for high performance applications. In suchapplications, a high f_(T) is required while maximizing the intrinsicgain. Hence, FIG. 6 illustrates g_(m)/g_(ds) at constant f_(T). In FIG.6, curve 1 represents a long channel device and curve 4 illustrates ashort channel device showing degraded g_(m)/g_(ds). Curve 2 illustratesthe forward biased asymmetric device illustrating an improved intrinsicvoltage gain, using embodiments of the invention e.g. as shown in FIG.3. Curve 3 illustrates the reverse biased asymmetric device.

FIG. 6, which includes FIGS. 6 a-6 e, illustrates various structuralembodiments of the invention. In FIGS. 6 a and 6 d, the gate line 50includes three gates. Hence, the device 10 includes a first gate 90, asecond gate 95 and a third gate 105. Each gate length is individuallyoptimized. In FIG. 6 b, the gate line 50 shares two active areas 110 and120 forming two transistors with their own source 30 and drain 60regions. FIG. 6 c illustrates an embodiment, wherein the first gate 90and second gate 95 are connected from both ends. FIG. 6 e illustrates anembodiment in which the both the gate lengths and widths of the gatelines are individually optimized. Although in this embodiment, thesection with the larger gate length has a smaller width, otherembodiments may use a different combination. For example, the sectionwith the larger gate length may have a larger width as illustrated inFIG. 6 f. The larger width may be preferable in some embodiments, as thehigher total current flowing through the larger width transistor resultsin a decrease in output conductance g_(ds) of the shorter widthtransistor. Consequently improving the intrinsic voltage gaing_(m)/g_(ds) of the combined devices.

A method of fabrication is illustrated in flow chart of FIG. 7 alongwith cross sectional views in FIG. 8, in accordance with an embodimentof the invention.

The analog device flow essentially comprises changing a digitaltechnology flow to accommodate the analog device features. Hence, onlysome processes pertaining to the analog device will be illustrated inFIGS. 7 and 8.

FIG. 8 a is a top cross sectional view and FIG. 8 b is a vertical crosssection. Referring first to FIGS. 8 a and 8 b and the flow chart of FIG.7, a semiconductor substrate 100 is provided. In the preferredembodiment, the semiconductor substrate 100 is a silicon wafer. Someexamples of the semiconductor substrate 100 are a bulk mono-crystallinesilicon substrate (or a layer grown thereon or otherwise formedtherein), a layer of a silicon-on-insulator (SOI) wafer.

Isolation trenches 28 are formed in the semiconductor substrate 100using conventional techniques. The isolation trenches 28 are then filledwith an isolating material forming isolation 40. The isolation processdefines the widths W of the transistors (to be formed).

The well implants, isolation implants, and threshold adjust implants arenext implanted. The analog regions are masked separately and implantedseparate from the logic regions. Some of the implants may be tailored tominimize noise from other active regions. For example, noise from otheractive devices may be transferred through the substrate. These signalscould be generated anywhere in the chip, for example in regions wherethe drains and collectors are capacitively coupled to the substrate.These noises could be shielded by adding sufficient isolation regions.In various embodiments, the isolation implants could be modified for theanalog device to shield the device from such external noise.

Referring next to FIG. 8 c, a gate dielectric 24 is deposited overexposed portions of the semiconductor substrate 100. Advanced deeplyscaled CMOS nodes have considerable gate leakage due to the use ofultra-thin gate dielectrics. For example, gate leakage can be of theorder of 1-10 nA/μm for thin gate oxides. Analog circuits cannottolerate such high gate leakages. For low-noise analog circuits, thegate leakage current level should be controlled. CMOS devices near thescaling limit are not suitable for low noise analog circuits. Also,analog circuit designers prefer to design with power-supply voltagesmuch larger than that of high-speed digital CMOS. However, most CMOSnodes include multiple gate oxide technologies especially for designinghigh voltage transistors such as Input/Output (110) devices. A thickgate oxide is chosen from available oxides to minimize gate tunnelingleakage. Of course, the thick-oxide devices are slower than thethin-oxide ones. In one embodiment, the gate dielectric 24 comprises anoxide (e.g., SiO₂), a nitride (e.g., Si₃N₄), or a combination of oxideand nitride (e.g., SiON, or an oxide-nitride-oxide sequence).

The gate electrode layer 26 is deposited over the gate dielectric 24.The gate electrode 26 preferably comprises a semiconductor material,such as polysilicon or amorphous silicon, although alternatively, othersemiconductor materials or metal may be used. The gate electrode layer26 having a thickness of between about 400 Å to 2000 Å may be depositedusing CVD, PVD, ALD, or other deposition techniques.

A resist layer 311 is deposited over the gate electrode layer 26. Theresist layer 311 may be either organic or inorganic. Some examples ofinorganic resist layer 311 include silicon dioxide, silicon nitride,silicon oxy-nitride, titanium nitride and/or a SILK (silicon-containinglow-k) layer. The resist layer 311 may also be an organic layer such asa bottom anti-reflective coating (BARC) layer (such as polymides, andpolysulfones), a FLARE layer, and/or a BCB layer. The resist layer 311may optionally be baked to form a hard baked, thermally or chemicallycross-linked resist. Finally, although only a single layer of resist 311is shown, the resist layer 311 may comprise multiple layers. Forexample, in some embodiments, the resist layer 311 may be a bilayer ortrilayer film comprising different materials.

A photo-resist 315 is deposited on the gate stack. The photo-resistlayer 315 is a resist that can be developed by exposure to radiationsuch as deep UV radiation used by lithography systems. In preferredembodiments, this photo-resist 315 is sensitive to 243 nm, 193 nm or 157nm electromagnetic radiation. The resist used may either be positive ornegative. Examples of resist polymers are poly-p-hydroxystyrene,acrylates, novolak or cycloaliphatic copolymers.

Referring to FIG. 8 d, the photo-resist 315 is exposed using a masklevel comprising the desired gate features (first and second gates 90and 95). The exposed photo-resist regions are removed by etching. Usingthe remaining photo-resist regions as patterns, an anisotropic etch suchas a reactive ion etch is used to remove the exposed portion of theresist 311. The vertical cross section at this stage of the fabricationprocess is illustrated in FIG. 8 e. Subsequent processing proceeds as ina conventional CMOS flow, forming drain extensions, source/drains usinganneals such a spike and/or laser anneals.

Embodiments of the invention will now be described to illustrateapplications of the invention in manufacturing an integrated mixedsignal system on chip (SoC) integrated circuits. SoC comprise bothanalog and digital components. As detailed in various embodiments aboveboth the digital and analog components are manufactured using a commonprocess. Embodiments illustrated in FIGS. 9 and 10 describe the finalSoC manufactured using such a common process.

Referring to FIG. 9 a, the SoC device 11 comprises digital and analogdevices. The digital devices include CMOS devices. For example, in FIG.9 a, the digital devices include a nMOS 207 and a pMOS 208. The SoCdevice 11 also includes the analog device 10. The nMOS 20, the pMOS 208and the analog device 10 are formed in a substrate 100 and comprisesource 30, drain 60, separated by gate line 50 and spacers 51. The nMOS207 comprises a p-channel 92 and the pMOS 208 comprises an n-channel 93.The analog device 10 includes a first transistor 900 and a secondtransistor 950. First channel 91 and second channel 96 are formed in thefirst transistor 900 and a second transistor 950 respectively. Thesource of the first transistor 900 is connected to the source 30; andthe drain of the second transistor 950 is connected to the drain 60. Thedrain of the first transistor 900 and the source of the secondtransistor 950 are connected via the shared source/drain 31. Thepotential difference applied between the source contact 80 and draincontact 70 determines the direction of the current flow (shown by thearrows).

In various embodiments of the invention, the threshold voltage of thefirst transistor 900 is higher compared to a threshold voltage of thesecond transistor 950. When the drain 60 of the device 10 is biasedusing the contact 70, the lower threshold voltage of the secondtransistor 950 results in an increased potential on the sharedsource/drain 31. Consequently, the first transistor 900 sees a higherdrain voltage resulting in a decrease in output conductance g_(ds) ofthe first transistor 900 reducing the overall output conductance g_(ds)of the combined first and second transistors 900 and 950. The reducedoverall output conductance g_(ds) of the combined first and secondtransistors 900 and 950 translates into a higher analog gain.

In various embodiments, the nMOS 20, the pMOS 208 and the analog device10 are manufactured preferably using the same process, for example, asdescribed in FIGS. 7 and 8. In different embodiments, the difference inthreshold voltages between first and second transistors 900 and 950 isobtained by a difference in gate lengths between the transistors. Invarious embodiments, this difference may also be obtained by using thehigh voltage or low voltage oxide, or alternately the low Vt or high Vtdrain extension (e.g., halo) implants. Note that neither of theseprocesses require additional masking steps. E.g. process flow for logicmanufacturing already includes multiple gate oxides, for example, forI/O devices and core devices. The high voltage oxide for forming I/Odevices can be used for the first transistor 900 and the low voltageoxide for forming the core devices can be used for the second transistor950. Similarly, different Vt masks are already in use in logicprocessing. A higher halo dose (e.g., used in the high Vt cmos device)can be used in forming the first transistor 900 and low halo dose (e.g.used in low Vt or nominal Vt cmos device) can be used in forming thesecond transistor 950.

The threshold voltage of the first transistor 900 is preferably about200 mV higher than the threshold voltage of the second transistor 950.In various embodiments, the threshold voltage of the first transistor900 is at least about 100 mV higher than the threshold voltage of thesecond transistor 950. Although in other embodiments, the thresholdvoltage of the first transistor 900 may be about 100 mV to about 300 mVhigher than the threshold voltage of the second transistor 950.

FIG. 9 b illustrates threshold voltage of a transistor (V_(T)) versusinverse gate length (1/L) of devices manufactured simultaneously using agiven CMOS process. For example, the digital devices (nMOS 207 and pMOS208) in FIG. 9 a may be formed using the shorter channel devices, andtypically are formed around the minimum gate length device. Asillustrated in FIG. 9 b, the first transistor 900 with a higherthreshold voltage may be selected from short channel devices, whereasthe second transistor 950 with the lower threshold voltage may beselected from long channel devices. The first transistor 900 ispreferably positioned around the peak of the threshold voltage versus1/L curve. FIG. 9 b illustrates the absolute threshold voltage. Hence,if both devices are pFET's, the first transistor 900 is more negativethan the second transistor 950.

FIG. 10, which includes FIGS. 10 a and 10 b illustrates a SoCimplementing multiple gate analog devices. Unlike FIG. 9 a, in FIG. 10 athe analog device 10 comprises three transistors: a first transistor900, a second transistor 950, and a third transistor 1050. As in theprevious embodiments, the threshold voltage of the first transistor 900is greater than the threshold voltage of the second transistor 950.Also, the threshold voltage of the second transistor 950 is greater thanthe threshold voltage of the third transistor 1050. One way ofimplementing the transistors with different threshold voltages is byusing transistors of different gate lengths. In this case, care shouldbe taken that the first transistor 900 is not made so short that itsthreshold voltage becomes lower than the second transistor 950 due toshort channel effects. For example, as illustrated in FIG. 10 b, therange of the first transistor 900 may be minimized or centered aroundthe peak of the threshold voltage rolloff. In various embodiments, thefirst transistor 900 may be larger than the minimum allowed gate length(L_(min)) or the gate length at which most of the digital devices aremanufactured. Although not discussed, in various embodiments, the analogdevice 10 may comprise more than three transistors.

Embodiments of the invention will now be described to illustrateapplications of the invention to analog circuits in FIGS. 11 and 12.FIG. 11, which includes Figures 11 a-11 c, illustrates op-amp circuitsincorporating embodiments of the invention. FIG. 12, which includes FIG.12 a and FIG. 12 b, illustrates current minors, in accordance withembodiments of the invention. Although, only a few analog circuits aredescribed here, embodiments of the invention apply to all analogcircuits.

FIG. 11 a illustrates an effective circuit, and FIGS. 11 b-11 cillustrate the complete circuit for a two-stage CMOS op-amp, inaccordance with an embodiment of the invention. The circuit consists ofan input differential stage with active load followed by a common-sourcestage, also with active load. The amplifier is required to drive a fixedon-chop capacitive load of a few picofarads. This amplifier is a typicalop-amp used in a large number of semiconductor components. For example,it is used applications in switched capacitor filters, sensing circuits,analog to digital converters etc.

In one embodiment, the op-amp is designed by optimizing the effectivecircuit of FIG. 11 a. For example, the widths (W₁-W₈) and lengths(L₁-L₈) of the individual transistors (M₁-M₈), as well as thecompensation capacitor C_(c) and resistance R_(c), and bias currentI_(b) may be optimized. The design parameters also include the voltagesV_(SS) and V_(DD). During analog device design the widths (W₁-W₈) andlengths (L₁-L₈) are determined. However, the optimized individualtransistors (M₁-M₈) may require very high analog gains unachievableusing a shared CMOS process. Alternately, in some embodiments, theoptimized large gate lengths may be larger than the maximum allowed gatelength in a CMOS technology.

FIG. 11 b illustrates an embodiment of the invention wherein each of thetransistors in FIG. 11 a is replaced by two or more transistors ofsmaller gate lengths. Hence, the transistors M₁-M₈ comprises transistors(M₁₁, M₁₂)-(M₈₁, M₈₂) with corresponding gate lengths

(L₁₁, L₁₂)-(L₈₁, L₈₂). The gate lengths (L₁₁, L₁₂)-(L₈₁, L₈₂) aresmaller than the corresponding gate lengths of the single gatetransistors illustrated in FIG. 11 a. The sets of transistors, forexample, (M₁₁, M₁₂) representing M₁, together match an expected analogvoltage gain, a goal previously unobtainable without altering themanufacturing process.

Although, in FIG. 11 b, all of the transistors in FIG. 11 a are replacedwith two smaller transistors, other embodiments may replace only some ofthe transistors. For example, after analog design, only some of thetransistors may require high analog voltage gain. For example, a voltagegain of about 50 or higher may be required by only some devices. In suchembodiments, only those transistors are replaced with two or moretransistors. For example, in FIG. 11 c, only the transistor M₈ of FIG.11 a is replaced with transistors M₈₁₁, M₈₂₁ and M₈₃₁.

FIG. 12 a illustrates an effective current mirror, and FIG. 12 billustrates the complete current mirror, in accordance with embodimentsof the invention. The effective current minor illustrated in FIG. 10 aincludes effective transistors M₁₁, and M₁₃, and transistor M₁₂. Thetransistor M₁₂ has a shorter gate length L₁₂ than the gate length L₁₁ ofthe other two effective transistors (M₁₁ and M₁₃).

FIG. 12 b illustrates the complete current mirror of FIG. 12 a. As theanalog voltage gain of the transistor M₁₁ with gate length L₁₁ is high,the transistor M₁₁ is replaced with other two transistors (M₁₁₁ andM₁₁₂) with an effective larger voltage gain. Similarly, if the requiredvoltage gain of the transistor M₁₃ with gate length L₁₁ is high, thetransistor M₁₃ is replaced with other two transistors (M₁₃₁ and M₁₃₂).In various embodiments, the effective transistors M₁₁ and M₁₃ aredivided into transistors with gate lengths L₁₁₁ and L₁₁₂, wherein thetransistor M₁₃₂ has a lower threshold voltage than the transistor M₁₃₁.In some embodiments, the gate length L₁₁₂ of the transistor M₁₃₂ islarger than the gate length L₁₁₁ of the transistor M₁₁₁. The gates ofthe transistors M₁₁₁ and M₁₁₂ are tied together forming the effectivetransistor M. Similarly, the gates of the transistors M₁₃₁ and M₁₃₂ aretied together forming the effective transistor M₁₃. The source/drain ofthe transistors M_(112 and) M₁₃₂ are tied together forming the currentmirror circuit.

Although embodiments of the present invention and their advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, it will be readily understood by those skilled in the artthat many of the features, functions, processes, and materials describedherein may be varied while remaining within the scope of the presentinvention.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A method of designing an integrated circuit, themethod comprising: designing a first transistor comprising a firstthreshold voltage, a first gate length and a first width; designing asecond transistor comprising a second threshold voltage, a second gatelength and a second width, wherein the first gate length is smaller thanthe second gate length, wherein the first threshold voltage is higherthan the second threshold voltage; designing an asymmetric transistor bycombining the first transistor and the second transistor, wherein thecombining comprises electrically coupling a drain of the firsttransistor and a source of the second transistor and electricallycoupling a gate of the first transistor with a gate of the secondtransistors, wherein, under a forward bias, the asymmetric transistorhas a voltage gain higher than a symmetric long channel transistorhaving a gate length longer than a sum of the first and the second gatelengths.
 2. The method of claim 1, further comprising replacing asymmetric transistor in a current mirror circuit with the asymmetrictransistor.
 3. The method of claim 1, further comprising replacing asymmetric transistor in a CMOS amplifier circuit with the asymmetrictransistor.
 4. The method of claim 1, wherein, under a reverse bias, theasymmetric transistor has a voltage gain comparable to the symmetriclong channel transistor.
 5. The method of claim 1, wherein the drain ofthe first transistor and the source of the second transistor share acommon region, and wherein the gate of the first transistor isphysically coupled to the gate of the second transistor.
 6. A method ofdesigning an integrated circuit, the method comprising: identifying atarget transistor with high analog gain in the integrated circuit;designing a first transistor comprising a first threshold voltage, afirst gate length and a first width; designing a second transistorcomprising a second threshold voltage, a second gate length and a secondwidth; replacing the target transistor with the first transistor and thesecond transistor, wherein a drain of the first transistor and a sourceof the second transistor are electrically coupled, and wherein a gate ofthe first transistor is electrically coupled to a gate of the secondtransistor; calculating an output analog voltage gain of the combinedfirst and second transistor; determining an error between the calculatedoutput analog voltage gain and a target analog voltage gain; andredesigning the first threshold voltage and a second threshold voltageto lower the error below an acceptable level, wherein after theredesigning, the first threshold voltage is higher than the secondthreshold voltage.
 7. The method of claim 6, wherein the first thresholdvoltage is higher than the second threshold voltage by at least about100 mV, and wherein the first and second transistors are of a same type.8. The method of claim 6, wherein redesigning the first thresholdvoltage and the second threshold voltage comprises redesigning the firstgate length and the second gate length, wherein after the redesigning,the first gate length is smaller than the second gate length.
 9. Themethod of claim 8, wherein the first and second gate lengths are lessthan 200 nm.
 10. The method of claim 6, wherein the drain of the firsttransistor and the source of the second transistor share a commonregion, wherein the gate of the first transistor is physically coupledto the gate of the second transistor.
 11. The method of claim 6, whereinredesigning the first threshold voltage and a second threshold voltageto lower the error below an acceptable level comprises reducing anoverall output conductance g_(ds) of the combined first and secondtransistors to achieve a higher calculated output analog voltage gain.12. The method of claim 6, wherein redesigning the first thresholdvoltage and the second threshold voltage comprises using a high voltagegate dielectric for the first transistor and a low voltage gatedielectric for the second transistor.
 13. The method of claim 6, whereinredesigning the first threshold voltage and the second threshold voltagecomprises using a higher halo dose for the first transistor and a lowerhalo dose for the second transistor.
 14. The method of claim 6, whereinthe integrated circuit is manufactured in a CMOS process common to thedigital devices.
 15. A method of designing an integrated circuit, themethod comprising: designing a first transistor comprising a firstthreshold voltage, a first gate length and a first width; designing asecond transistor comprising a second threshold voltage, a second gatelength and a second width; designing an asymmetric transistor bycombining the first transistor and the second transistor, wherein thecombining comprises electrically coupling a drain of the firsttransistor and a source of the second transistor and electricallycoupling a gate of the first transistor with a gate of the secondtransistors; calculating an output analog voltage gain of the asymmetrictransistor; determining an error between the calculated output analogvoltage gain and a target analog voltage gain; and redesigning the firstthreshold voltage and a second threshold voltage to lower the errorbelow an acceptable level, wherein after the redesigning, the firstthreshold voltage is higher than the second threshold voltage, whereinredesigning the first threshold voltage and the second threshold voltagecomprises redesigning the first gate length and the second gate length,wherein after the redesigning, the first gate length is smaller than thesecond gate length.
 16. The method of claim 15, wherein the asymmetrictransistor is part of a current minor circuit.
 17. The method of claim15, wherein the asymmetric transistor is part of a CMOS amplifiercircuit.
 18. The method of claim 15, wherein the integrated circuit ismanufactured in a CMOS process common to the digital devices.
 19. Themethod of claim 15, further comprising replacing a symmetric transistorin an analog component of the integrated circuit with the asymmetrictransistor.
 20. The method of claim 15, wherein the drain of the firsttransistor and the source of the second transistor share a commonregion, and wherein the gate of the first transistor is physicallycoupled to the gate of the second transistor.